The R-T-B based sintered magnet (R represents a rare earth element, T represents one or more elements of iron group elements containing Fe as an essential, and B represents boron), a representative of which is Nd—Fe—B based sintered magnet, is advantageous for miniaturization and high efficiency of the machines using it due to high saturation flux density, and thus can be used in the voice coil motor of the hard disk drive and the like. In recent years, the R-T-B based sintered magnet also has been applicable in various industrial motors, or driving motors of hybrid vehicles, or the like. From the viewpoint of energy conservation and the like, it is desirable that the R-T-B based sintered magnet can be further popularized in these fields. However, when applied in the hybrid vehicles and the like, the R-T-B based sintered magnet will be exposed to a relatively high temperature. Therefore, inhibition of the high temperature demagnetization caused by heat becomes important. For inhibition of the demagnetization under high temperature, a method for sufficiently improving coercivity of the R-T-B based sintered magnet at room temperature is well known as effective.
For example, as a method for improving a coercivity of the Nd—Fe—B based sintered magnet at room temperature, a method in which part of Nd of the compound Nd2Fe14B as the main phase is replaced with heavy rare earth elements such as Dy, Tb and the like is well known. By replacing part of Nd with the heavy rare earth elements, the magnetic anisotropy of crystals is increased, and as a result, the coercivity of the Nd—Fe—B based sintered magnet at room temperature can be sufficiently improved. In addition to the replacement with heavy rare earth elements, addition of elements such as Cu and the like is also effective in improving coercivity at room temperature (Patent Document 1).
As a method for improving coercivity by controlling the microstructure, i.e., the structures of the R2T14B main phase crystal grains of the rare earth based magnet and the grain boundary phases, the technique disclosed in Patent Document 2 is known. Patent Document 2 discloses a rare earth based magnet which is a magnetic body including main phase crystal grains of the tetragonal intermetallic compound (R2Fe14B) and grain boundary phases formed among the crystal grains. The crystal grains have a round-corner shape with a maximum width of 500 nm, and the grain boundary phases have a minimum width of 1 nm or more. Here, the maximum width means the maximum length of a crystal grain in the direction perpendicular to the easy magnetization axis (axis c) shown in the section shape when the main phase crystal grain is cut by a plane parallel to axis c. Moreover, the round-corner shape means that any angular part is not found in the section shape. That is, the above round-corner shape means, unlike a rectangular shape of the existing crystal grains, the angles forming the rectangle become substantially arc-shaped. It is considered that by means of forming such a structure, the influence of the demagnetizing field at the angles of the crystal grains is reduced, resulting in inhibition of the occurrence of reverse magnetic domain in the crystal grains and the like, and improvement of a coercivity of the magnetic body.